Re: [PATCH for-next] RDMA/efa: Expose device P2P DMA support via device query

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 24, 2026 at 03:29:13PM +0000, Michael Margolin wrote:

> > > 2. Expose EFA_DEV_CAP(dev, PCIE_PEER_ACCESS_ENABLED)
> > >    directly to userspace. As you already stated, this is a
> > >    bit awkward as a device isn't expected to be aware of
> > >    system topology, but since EFA lives in the cloud it is
> > >    partially aware of the platform configuration and in
> > >    particular the ability to access peer devices over PCIe.
> > 
> > If you already know your VMs are always safe what is even the issue?
> > What are you probing for?
> > 
> > Lets please not hack around bad userspace with even worse kernel uAPI.
> 
> The EFA device already knows this attribute. The problem is that this
> information is not exposed to userspace today.
> 
> So if I go back to our original proposal - exposing the
> EFA_DEV_CAP(dev, P2P_DMA) bit through query device - this is a firmware
> attribute that says "this device does not block PCIe peer access on its
> end." It's not a topology claim, it's a device property, similar to how
> we already expose RDMA read/write support.

And how does that help you if you actually need reachability? You
haven't said directly what you are even trying to achieve (though I
have a pretty good guess).

If all your VMs can do P2P then what is the issue? If you need
pairwise reachability then you have to use dmabuf.

Precompute it and use a topology file like everyone else?

Inject a topology file into your VMs, like some others?

Jason